Electron transport layer over an inorganic photoconductive layer

ABSTRACT

An electrophotographic plate comprising two adjacent layers, one of which is a photoconductive layer capable of photogenerating and injecting electrons into the other contiguous layer, which is an electronically active material capable of supporting electron injection and transport. The electronically active transport material has the additional property of being substantially transparent to radiation in the particular wavelength region of xerographic use thereby rendering it particularly useful as a relatively thick protective overlayer for the photoconductive portion of the plate. The structure may be imaged in the conventional xerographic mode which usually includes charging, exposure to light, and development.

United States Patent [191 Regensburger Dec. 23, 1975 ELECTRON TRANSPORTLAYER OVER AN INORGANIC PHOTOCONDUCTIVE LAYER [75] Inventor: Paul J.Regensburger, Webster,

[21] Appl. No.: 341,839

Related US. Application Data [63] Continuation-impart of Ser. No.94,071, Dec. 1, 1970, abandoned, which is a continuation-in-part of Ser.No. 14,282, Feb. 26, 1970, abandoned.

[52] US. Cl. .Q 96/15; 96/l.8 [51] Int. Cl. G03G 5/08; GO3G 13/22 [58]Field of Search 96/1.5, 1.8; 252/501 [56] References Cited UNITED STATESPATENTS 2,901,348 8/1959 Dessauer et a1. 96/1.5 3,287,123 11/1966 Hoege96/1.5 3,408,189 10/1968 Mammino.. 96/1.5

3,573,906 4/1971 Goffe 96/1.8 3,595,771 7/1971 Weigl 96/1.5 X 3,598,5828/1971 Herrick et al 96/].5

3,634,079 l/l972 Champ et al. 96/l.5 3,725,058 4/1973 Hayashi et al.96/].5

FOREIGN PATENTS OR APPLICATIONS 4,316,198 7/1968 Japan 96/].5

Primary Examiner-Roland E. Martin, Jr. Attorney, Agent, or Firm-James J.Ralabate; James P. OSullivan; Donald M. MacKay [5 7] ABSTRACT Anelectrophotographic plate comprising two adjacent layers, one of whichis a photoconductive layer capable of photogenerating and injectingelectrons into the other contiguous layer, which is an electronicallyactive material capable of supporting electron injection and transport.The electronically active transport material has the additional propertyof being substantially transparent to radiation in the particularwavelength region of xerographic use thereby rendering it particularlyuseful as a relatively thick protective overlayer for thephotoconductive portion of the plate. The structure may be imaged in theconventional xerographic mode which usually includes charging, exposureto light, and development.

10 Claims, 6 Drawing Figures U.S. Patent Dec.23, 1975 Sheetlof23,928,034

FIG ,2

FIG. 2

US. Patent Dec. 23, 1975 Sheet2of2 3,928,034

FIG 4 FIG 6 ELECTRON TRANSPORT LAYER OVER AN INORGANIC PHOTOCONDUCTIVELAYER BACKGROUND OF THE INVENTION This application is acontinuation-in-part of my previous application, Ser. No. 94,071, filedDec. 1, 1970, now abandoned, which is a continuation-in-part of Ser. No.14,282, filed Feb. 26, 1970, now abandoned.

This invention relates in general to xerography and more specifically toa novel photosensitive device and method of use.

In the art of xerography, a xerographic plate containing aphotoconductive insulating layer is imaged by first uniformlyelectrostatically charging its surface. The plate is then exposed to apattern of activating electromagnetic radiation such as light, whichselectively dissipates the charge in the illuminated areas of thephotoconductive insulator while leaving behind a latent electrostaticimage in the non-illuminated areas. This latent electrostatic image maythen be developed to form a visible image by depositing finely dividedelect'roscopic marking particles on the surface of the photoconductiveinsulating layer.

A photoconductive layer for use in xerography may be a homogeneous layerof a single material such as vitreous selenium or it may be a compositelayer containing a photoconductor and another material. One type ofcomposite photoconductive layer used in xerography is illustrated byU.S. Pat. No. 3,121,006 to Middleton and Reynolds which describes anumber of binder layers comprising finely-divided particles of aphotoconductive inorganic compound dispersed in an electricallyinsulating organic resin binder. In its present commercial form, thebinder layer contains particles of zinc oxide uniformly dispersed in aresin binder and is coated on a paper backing.

In the particular examples of binder systems described in Middleton etal., the binder comprises a material which is incapable of transportinginjected charge carriers generated by the photoconductor particles forany significant distance. As a result, with the particular materialsdisclosed in the Middleton et al. patent, the photoconductor particlesmust be in substantially continuous particle-to-particle contactthroughout the layer in order to permit the charge dissipation requiredfor cyclic operation. With the uniform dispersion of photoconductorparticles described in Middleton et a1, therefore, a relatively highvolume concentration of photoconductor, up to about 50 percent or moreby volume, is usually necessary in order to obtain sufficientphotoconductor particle-to-particle contact for rapid discharge. It hasbeen found, however, that high photoconductor loadings in the binderlayers of the resin type result in the physical continuity of the resinbeing destroyed, thereby sufficiently reducing the mechanical propertiesof the binder layer. Layers with high photoconductor loadings are oftencharacterized by a brittle binder layer having little or no flexibility.On the other hand, when the photoconductor concentration is reducedappreciably below about 50 percent by volume, the discharge rate isreduced, making high speed cyclic or repeated imaging difficult orimpossible.

U.S. Pat. No. 3,121,007 to Middleton et a1. teaches another type ofphotoconductor which includes a two phase photoconductive binder layercomprising photoconductive insulating particles dispersed in ahomogeneous photoconductive insulating matrix. The photoconductor is inthe form of a particulate photoconductive inorganic crystalline pigmentbroadly disclosed as being present in an amount from about 5 to percentby weight. Photodischarge is'said to be caused by the combination ofcharge carriers generated in the photoconductive insulating matrixmaterial and charge carriers injected from the photoconductivecrystalline pigment into the photoconductive insulating matrix.

U.S. Pat. No. 3,037,861 to Hoegl et al. teaches that polyvinyl carbazoleexhibits some long-wave U.V. sensitivity and suggests that its spectralsensitivity be extended into the visible spectrum by the addition of dyesensitizers. Hoegl et a1. further suggests that other additives such aszinc oxide or titanium dioxide may also be used in conjunction withpolyvinyl carbazole. In Hoegl et al., it is clear that the polyvinylcarbazole is intended to be used as a photoconductor, with or withoutadditives materials which extend its spectral sensitivity.

In addition, certain specialized layer structures particularly designedfor reflux imaging have been proposed. For example, U.S. Pat. No.3,165,405 to Hoesterey utilizes a two layered zinc oxide binderstructure for reflex imaging. The Hoesterey patent utilizes two separatecontiguous photoconductive layers having different spectralsensitivities in order to carry out a particular reflex imagingsequence. The Hoesterey device utilizes the properties of multiplephotoconductive layers in order to obtain the combined advantages of theseparate photoresponse of the respective photoconductive layers.

It can be seen from a review of the conventional compositephotoconductive layers cited above, that upon exposure to light,photoconductivity in the layer structure is accomplished by chargetransport through the bulk of the photoconductive layer, as in the caseof vitreous selenium (and other homogeneous layer modifications). Indevices employing photoconductive binder structures, which includeinactive electrically insulating resins such as those described in theMiddleton et al., U.S. Pat. No. 3,121,006, conductivity or chargetransport is accomplished through high loadings of the photoconductivepigment allowing particle-toparticle contact of the photoconductiveparticles. In the case of photoconductive particles dispersed in aphotoconductive matrix, such as illustrated by the Middleton et al.,U.S. Pat. No. 3,121,007, photoconductivity occurs through the generationof charge carriers in both the photoconductive matrix and thephotoconductor pigment particles.

Although the above patents rely upon distinct mechanisms of dischargethroughout the photoconductive layer, they generally suffer from commondeficiencies in that the photoconductive surface during operation isexposed to the surrounding environment, and particularly in the case ofcycling xerography, susceptible to abrasion, chemical attack, heat, andmultiple exposures to light during cycling. These effects arecharacterized by a gradual deterioration in the electricalcharacteristics of the photoconductive layer resulting in the printingout of surface defects and scratches, localized areas of persistentconductivity which fail to retain an electrostatic charge, and high darkdischarge.

In addition to the problems noted above, these photoconductive layersrequire that the photoconductor comprise either a hundred percent of thelayer, as in the case of the vitreous selenium layer, or that theypreferably contain a high proportion of photoconductive material in thebinder configuration. The requirements of the photoconductive layercontaining all or a major proportion of a photoconductive materialfurther restricts the physical characteristics of the final plate, drumor belt in that the physical characteristics such as flexibility andadhesion of the photoconductor to a supporting substrate are primarilydictated by the physical properties of the photoconductor, and not bythe resin or matrix material which is preferably present in a minoramount.

Another form of composite photosensitive layer which has also beenconsidered by the prior art includes a layer of photoconductive materialwhich is covered with a relatively thick plastic layer and coated on asupporting substrate.

US. Pat. No. 3,041,166 to Bardeen describes such a configuration inwhich a transparent plastic material overlays a layer of vitreousselenium which is contained on a supporting substrate. The plasticmaterial is described as one having a long range for charge carriers ofthe desired polarity. In operation, the free surface of the transparentplastic is electrostatically charged to a given polarity. The device isthen exposed to activating radiation which generates a hole-electronpair in the photoconductive layer. The electron moves through theplastic layer and neutralizes a positive charge on the free surface ofthe plastic layer thereby creating an electrostatic image. Bardeen,however, does not teach any specific plastic materials which willfunction in this manner, and confines his examples to structures whichuse a photoconductor material for the top layer.

French Pat. No. 1,577,855 or U.S. Pat. No. 3,598,582 to Herrick et al.describes a special purpose composite photosensitive device adapted forreflex exposure by polarized light. One embodiment which employs a layerof dichroic organic photoconductive particles arrayed in orientedfashion on a supporting substrate and a layer of polyvinyl carbazoleformed over the oriented layer of dichroic material. When charged andexposed to light polarized perpendicularly to the orientation of thedichroic layer, the oriented dichroic layer and polyvinyl carbazolelayer are both substantially transparent to the initial exposure light.When the polarized light hits the white background of the document beingcopied, the light is depolarized, reflected back through the device andabsorbed by the dichroic photoconductive material. In anotherembodiment, the dichroic photoconductor is dispersed in oriented fashionthroughout the layer of polyvinyl carbazole.

In view of the state of the art, it can readily be seen that there is aneed for a general purpose photoreceptor exhibiting acceptablephotoconductive characteristics and which additionally provides thecapability of exhibiting outstanding physical strength and flexibilityto be reused under rapid cyclic conditions without the progressivedeterioration of the xerographic properties due to wear, chemicalattack, and light fatigue.

OBJECTS OF THE INVENTION It is therefore an object of the presentinvention to provide an electrophotographic plate adapted for cyclicimaging devoid of the above noted disadvantages.

Another object of this invention is to provide an electrophotographicplate having excellent abrasion resistance properties.

It is yet another object of this invention to provide a novel imagingsystem.

It is yet another object of the instant invention to provide anelectrophotographic plate having a material which exhibit facileelectron transport properties.

It is still another object of this invention to provide aphotoconductive insulating layer for an electrophotographic plate whichis both relatively easy to make and inexpensive.

SUMMARY OF THE INVENTION The foregoing objects and others areaccomplished in accordance with the present invention by providing anelectrophotographic plate having a novel two layered structurecomprising (a) a photoconductive layer capable of photogeneratinghole-electron pairs and injecting the electrons into the adjacentoverlayer, and (b) an adjacent electronically active transport materiallayer, which is substantially transparent and nonabsorbing in theparticular wavelength region of xerographic use, said electronicallyactive transport layer comprising an electron transport material insufficient concentration to be capable of accepting and transportingelectrons which have been injected from the photoconductive layer.

As defined herein, a photoconductor is a material which is electricallyphotoresponsive to light in the wavelength region in which it is to beused. More specifically, it is a material whose electrical conductivityincreases significantly in response to the absorption of electromagneticradiation in a wavelength region in which it is to be used. Thisdefinition is necessitated by the fact that a vast number of aromaticorganic compounds are known or expected to be photoconductive whenirradiated with strongly absorbed ultraviolet, x-ray, orgamma-radiation. Photocon'ductivity in organic materials is a commonphenomenon. Practically all highly conjugated organic compounds exhibitsome degree of photoconductivity under appropriate conditions. Most ofthese organic materials have their prime wavelength response in theultraviolet. However, little commercial utility has been found forultraviolet responsive materials, and their short wavelength response isnot particularly suitable for document copying or color reproduction. Inview of the general prevelance of photoconductivity in organic compoundsfollowing short wavelength excitation, it is therefore necessary thatfor the instant invention, the term photoconductor and photoconductivebe understood to include only those materials which are in factsubstantially photoresponsive in the wavelength region in which they areto be used.

In accordance with the present invention it has been found that axerographic or electrophotographic sensitive member can be prepared withelectronically active transport materials comprising aromatic orheterocyclic electron acceptors which facilitate the transport ofphotogenerated electrons from a photoconductive layer under theinfluence of an electric filed. The active transport materials, whichare also referred to as active matrix materials when used as matricesfor a binder layer, to be described herein, are to be distinguished fromthose matrix binders or the prior art, described above, in that thepresent materials have the combined properties of being substantiallytransparent, hence, non-photoconductive and non-absorbing, in at leastsome significant portion of a particular wavelength region ofxerographic use corresponding to a range of photosensitivity of thephotoconductor, and are capable of supporting the injection andtransport of electrons which are photogenerated in an adjacent layer ofphotoconductor. Because of their unique combination of substantialtransparency in a wavelength region of particular xerographic use andelectron transport capability, the active transport materials of thepresent invention can be used effectively as a relatively thickelectrically insulating overcoating of the photoconductive layer and yetfunction as both a window and a charge transport means for saidphotoconductive layer. These particular characteristics of the materialsused in the present invention enables use of a relatively small amountof photoconductor in the total photoconductive insulating layer.

It should be understood that the active transport layer does notfunction as a photoconductor in the wavelength region of use. As statedabove, hole-electron pairs are photogenerated in the photoconductivelayer and the electrons are then injected across a field modulatedbarrier into the active layer and electron transport occurs through theactive layer.

It is to be further noted that most materials which are useful foractive transport layers of the instant invention are incidentally alsophotoconductive when radiation of wavelengths suitable for electronicexcitation is absorbed by them. However, photoresponse in the shortwavelength region, which falls outside the spectral region for which thepresent photoconductors are to be used, is irrelevant to the performanceof the device. It is well known that radiation must be absorbed in orderto excite photoconductive response, and the transparency criterion,stated above, for the active transport materials implies that thesematerials do not contribute significantly to the photoresponse of thephotoreceptor in the wavelength region of use.

A typical application of this invention includes the use of a sandwichcell or layered configuration which in one embodiment consists of asupporting substrate, such as a conductor, having a photoconductivelayer coated thereon. The photoconductive layer may be in the form of alayer of amorphous or vitreous selenium. A layer of active transportmaterial which is substantially transparent in a significant portion ofthe particular wavelength region in which selenium is photoresponsiveand is coated over the selenium photoconductive layer. The use of theactive transport material allows one to take advantage of placing aphotoconductive layer adjacent to a supporting substrate and protectingsaid photoconductive layer with a protective overlayer or window whichwill allow for the transport of photo-excited electrons from theselenium layer and which can be of thickness sufficient to physicallyprotect the photoconductive layer from environmental conditions. Thisstructure can be imaged in the conventional xerographic manner whichincludes charging, exposure, and development. I

The use of the active transport concept of the present invention enablesone to use particular regions of the electromagnetic spectrum forselective xerographic copying. A typical application would be the use ofelectronically active materials in color xerography to copy particularcolor sequentially and thereby obtain a complete color print.

DESCRIPTION OF THE DRAWINGS Further objects of the invention, togetherwith additional features contributing thereto will be apparent from thefollowing description of one embodiment of the invention when read inconjunction with the accompanying drawings wherein:

FIG. 1 is a schematic sectional view of one embodiment of a xerographicdevice contemplated by the instant invention.

FIG. 2 illustrates a second embodiment of a xerographic device of theinstant invention.

FIG. 3 illustrates a third embodiment of a xerographic device of theinstant invention.

FIG. 4 illustrates a discharge mechanism of photodischarge of theelectronically active material layer.

FIG. 5 illustrates the discharge mechanism of one binder system of theprior art.

FIG. 6 illustrates the discharge mechanism of another binder system ofthe prior art.

DETAILED DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates one embodimentin improved xerographic plate 10 according to this invention. Referencecharacter 11 designates a substrate or mechanical support. The substratemay comprise a metal such as brass, aluminum, gold, platinum, steel orthe like. It may be of any convenient thickness, rigid or flexible, inthe form of j sheet, web, cylinder, or the like, and may be coated witha thin layer of plastic. It may also comprise such other materials asmetallized paper, plastic sheets covered with a thin coating of aluminumor copper iodide, or glass coated with a thin layer of chromium or tinoxide. It is usually preferred that the support member be somewhatelectrically conductive or have a somewhat conductive surface and thatit be strong enough to permit a certain amount of handling. In certaininstances, however, support ll need not be conductive or may even bedispensed with entirely.

Reference character 12 designates a photoconductive monolayer or unitarylayer, which comprises a photoconductive material which is capable ofphotogenerating electrons and injecting them into the overlaying activematrix material.

Generally any photoconductive material capable of photogeneratingelectrons will function with the electron transport materials of thepresent invention. Typical inorganic crystalline photoconductors includecadmium sulfide, cadmium sulfoselenide, cadmium selenide, zinc sulfide,zinc oxide, and mixtures thereof. The typical inorganic photoconductiveglasses include amorphous selenium, and selenium alloys such asseleniumtellurium, and selenium-arsenic. Selenium may also be used inits hexagonal crystalline form, commonly referred to as trigonalselenium. Typical organic photoconductors include phthalocyaninepigments such as the X-form of metal free phthalocyanine described inUS. Pat. No. 3,357,989 to Byme et al., and metal phthalocyaninepigments, such as copper phthalocyanine. Other typical organicphotoconductors include photoinjecting pigments such as benzimid azolepigments, perylene pigments, quinacridone pigments, indigoid pigmentsand polynuclear quinones all of which are disclosed in copendingapplicant Ser. No. 93,974; 94,066; 94,040; 94,067; 94,068; all filed onDec. 1, 1970 and all abandoned. The above list of photoconductors shouldin no way be taken as limiting but as merely illustrative of materialshaving particularly efficient electron injection properties.

Photoconductive monolayer 12 of FIG. 1 may be in any suitable thicknessused for carrying out its function in the xerographic insulating member.Typical thicknesses for this purpose range from 0.02 to 20 microns.Thicknessesabove 25 microns tend to produce undesirable positiveresidual buildup in the photoconductive layer during recycling andexcessive dark decay while thicknesses below 0.02 micron becomeinefficient in absorbing impinging radiation. A range of from about 0.2to microns is preferred since these thicknesses would ensure maximumfunctionality of the photoconductor with a minimum amount of saidphotoconductive substance and completely avoid the above mentionedproblems with regard to thickness. As pointed out above, one of theprimary advantages of the instant invention is the use of a minimumamount of photoconductor in the photoconductive insulating layer.

Reference character 13 designates the active transport material layerwhich overlays the photoconductive layer 12. As heretobefore mentioned,the active transport layer comprises an electron transport materialwhich is capable of both supporting electron injection from thephotoconductive layer and transporting said photogenerated electronsunder the influence of an applied field. In order to function in themanner outlined above, the active transport material should besubstantially transparent to the particular wavelength region used forxerographic copying. In particular, the active transport material shouldbe substantially nonabsorbing in at least a significant portion thatpart of the electromagnetic spectrum which ranges from about 4200 to8000 Angstroms becuase most xerographically useful photoconductors havephotoresponse to wavelengths in this region.

As mentioned above, the active transport layer 12 comprises aromatic orheterocyclic electron acceptor materials which have been found toexhibit negative charge carrier transport properties as well as therequisite transparency characteristics. Typical electron acceptormaterials within the purview of the instant invention include phthalicanhydride, tetrachlorophthalic anhydride, benzil, mellitic anhydride,S-tricyanobenzene, picryl chloride, 2,4-dinitrochlorobenzene,2,4-dinitrobromobenzene, 4-nitrobiphenyl, 4,4-dinitrobiphenyl,2,4,6-trinitroanisole, trichlorotrinitrobenzene, trinitro-O-toluene,4,6-dichloro-l, 3-dinitr0benzene, v 4,6-dibromol ,3-dinitrobenzene,P-dinitrobenzene, chloranil, bromanil, and mixtures thereof. It isfurther intended to include within the scope of those materials suitablefor the active transport layer, other reasonable structural or chemicalmodifications of the above described materials provided that themodified compound exhibits the desired charge carrier transportcharacteristics.

While any and all aromatic or heterocyclic electron acceptors having therequisite transparency characteristic are within the purview of theinstant invention particularly good electron transport properties arefound with aromatic or heterocyclic compounds having more than onesubstituent of the strong electron withdrawing substituents such asnitro-(-NO sulfonate ion (-80 carboxyl-(-COOH) and cyano-(CN) groupings.From this class of materials, 2,4,7-trinitro-9- fluorenone (TNF),2,4,5,7-tetranitrofluorenone, trinitroanthraccne, dinitroacridine,tetracyanopyrene, and dinitroanthraquinone are preferred materialsbecause of their availability and superior electron transportproperties.

It will be obvious to those skilled in the art that the use of anypolymer having the described aromatic or heterocyclic electron acceptormoiety as an integral portion of the polymer structure, will function asan active transport material. It is not the intent of the invention torestrict the type of polymer which can be employed as the transportmaterial, provided it has an active electron acceptor moiety to providethe polymer with electron transport characteristics. Polyesters,polysiloxanes, polyamides, polyurethanes, and epoxies, as well as block,random or graft copolymers containing the aromatic moiety are thereforeexemplary of the various types of polymers which could be employed. Inaddition, electronically inactive polymers in which the active electronacceptor material is dispersed at high concentration can be employed ashereinafter described.

The substantial or significant transparency of the active transportmaterial within the context of the instant invention, as exemplified byFIG. 1, means that a sufficient amount of radiation from a source mustpass through the active transport layer 13 in order that thephotoconductive layer 12 can function in its capacity as aphotogenerator and injector of electrons. More specifically, substantialtransparency is present in the active transport materials of the presentinvention when the active transport material is non-photoconductive andnon-absorbing in at least some significant portion of the wavelengthregion of from about 4200 to 8000 Angstrom Units. This property ofsubstantial transparency enables enough activating radiation to impingethe photoconductor layer so as to cause discharge of the charged activetransport photoreceptor of the present invention.

It is not the intent of this invention to strictly restrict the choiceof active transport materials to those which are transparent in theentire visible region. For example, when used with a transparentsubstrate, imagewise exposure may be accomplished through the substratewithout the light passing through the layer of active transportmaterial. In this case, the active material need not be non-absorbing inthe wavelength region of use. This particular application takesadvantage of the injection and transport properties of the presentactive materials and falls within the purview of the instant invention.Other applications where complete transparency is not required for theactive material include the selective recording of narrow-band radiationsuch asthat emitted from lasers, spectral pattern recognition, colorcoded form duplication, and possibly color xerography.

While the active material layer 13 of FIG. 1 may consist exclusively ofcharge transport material, for purposes of the present invention, thelayer may also comprise the charge transport material at a sufficientconcentration in a suitable electronically inert binder material toeffect particle-to-particle contact or to effect suflicient proximitythereby permitting effective charge transport from the photoinjectingpigments of the instant invention through the layer. Generally theremust be a volume ratio of at least 25 percent active transport materialto electronically inert binder material to obtain the desiredparticle-to-particle contact or proximity. Typical resin bindermaterials for the practice of the invention are polystyrene; siliconeresins such as DC-l, DC-804, and DC-996 all manufactured by the DowCorning Corporation; and Lexan, a polycarbonate resin, SR-82manufactured by the General Electric Company; acrylic and methacrylicester polymers such as Acryloid A10 and Acryloid B72, polymerized esterderivatives of acrylic and alpha-acrylie acids both supplied by Rohm andHaas Company and Lucite 44, Lucite 45 and Lucite 46 polymerized butylmethacrylates supplied by the E. I. duPont de Nemours & Company;chlorinated rubber such as Parlon supplied by the Hercules PowderCompany; vinyl polymers and copolymers such as polyvinyl chloride,polyvinyl acetate, etc. including Vinylite VYHH and VMCH manufactured bythe Bakelite Corporation; cellulose esters and ethers such as ethylcellulose, nitrocellulose, etc.; alkyd resins such as Glyptal 2469manufactured by the General Electric Company; etc. In addition, mixtureof such resins with each other or with plasticizers so as to improveadhesion, flexibility, blocking, etc. of the coating may be used. Thus,Rezyl 869 (a linseed oil-glycerol alkyd manufactured by AmericanCyanamid Company) may be added to chlorinated rubber to improve itsadhesion and flexibility. Similarly, Vinylites VYHH and VMCH (polyvinylchlorideacetate copolymers manufactured by the Bakelite Company) may beblended together. Plasticizers include phthalates, phosphates, adipates,etc. such as tricresyl phosphate, dicotyl phthalate, etc. as is wellknown to those skilled in the art.

The active transport material which is employed in conjunction with thephotoconductive layer in the instant invention is a material which is aninsulator to the extent that an electrostatic charge placed on saidactive transport material is not conducted in the absence ofillumination at a rate sufficient to prevent the formation and retentionof an electrostatic latent image thereon. In general, this means thatthe specific resistivity of the active transport material should be atleast 10 ohms-cm. and preferably will be several orders higher. Foroptimum results, however, it is preferred that this specific resistivityof the active transport material be such the the overall resistivity ofthe active binder layer in the absence of activating illumination orcharge injection from an adjacent layer be about 10 ohm-cm.

Because the overlayer functions as an active transport layer thicknessis not critical to the function of the xerographic member. However, thethickness of said active transport layer would be dictated by practicalneeds in terms of the amounts of electrostatic charge necessary toinduce an applied field suitable to effect electron injection andtransport. Active transport layer thicknesses of from about 5 to 100microns would be suitable, but thicknesses outside this range may beused. The ratio of the thickness of the active transport layer to thephotoconductive layer should be maintained from about 2:1 to 200:1.

Another modification of the layered structure of FIG. 1 is illustratedin FIG. 2 where the photoconductive layer is depicted as being a layerof binder material having crystalline particles of photoconductordispersed therein. The binder material may be any suitable organicsubstance used for such purposes including inert binder materials or oneof the active matrix materials of the instant invention. Theconcentration of the photoconductor material will vary according towhich type of binder material is used and will range in value from about5-99 volume percent of the total photoconductive layer. If anyelectronically inert binder material is used in combination with thephotoconductor material a volume fraction of at least 25 percentphotoconductor to the electronically inert binder material is necessaryto effect particle-to-particle contact or proximity thereby renderinglayer l2 photoconductive throughout. The remarks with regard to thethickness of the photoconductive layer of FIG. 1 are generallyapplicable here; namely, a range of from about 0.05 to 20 microns, witha range of 0.3 to 5 microns being preferred due to the excellent resultsderived from this thickness range. The size of the photoconductiveparticles in the binder layer is not particularly critical, butparticles in a size range of about 0.01 to 1.0 microns yieldparticularly satisfactory results.

Another variation of the layered configuration described in FIGS. 1 and2 consists of the use of a blocking layer 14 at thesubstrate-photoconductor interface said layer being illustrated in FIG.3 This blocking layer aids in sustaining an electric field across thephotoconductor-active organic layer after the charging step. Anysuitable blocking material may be used. Typical materials include nylon,epoxy, aluminum oxide, and insulating resins of various types includingpolystyrene, butadiene polymers and copolymers, acrylic and methacrylicpolymers, vinyl resins, alkyd resins, and cellulose base resin.

It can therefore be seen that the photo-insulating portion of thexerographic members of the instant invention represent in FIGS. l-3 isderived into two functional layers:

1. A photoconductive layer which photogenerates holes and electrons uponexcitation by radiation and injects said photogenerated electrons intothe overlaying electronically active transport material, and;

2. An overlaying substantially transparent active transport materialwhich allows transmission of radiation to the photoconductive layer,accepts the subsequently photogenerated electrons from thephotoconductor material, and actively transports aid conduction electronto its positively charged surface thereby neutralizing said charge.

This is more dramatically illustrated in FIG. 4 where the xerographicmember of the present invention has been positively charged by means ofcorona charging. The light 14 represented by the arrows then passesthrough the transparent active transport layer and impinges thephotoconductive layer thereby creating a hole-electron pair. Theelectron and hole are then separated by the force of the applied fieldand the electron injected across the interface into the active transportlayer where it is then transported by force of the electrostaticattraction through the active transport layer system' to the surfacewhere it neutralizes the positive charge previously deposited by meansof corona charging. Since only photogenerated electrons can move in theelectron transport active material layer, large changes in surfacepotential result only when the electric field in the layer is such as tomove the photogenerated electrons from the photoconductor layer wherethey are generated, through the active matrix layer and then to thecharged surface. This means that for maximum utility the active matrixlayer must be charged positively.

Referring now to FIG. 5 there is illustrated an electrophotographicplate of the prior art in which sensitizing pigment 12 has beendispersed in a photoconductor binder material 13 for the purpose ofincreasing the sensitivity of said photoconductor material. The light 14impinges the electrophotographic member and creates photogenerated holesand electrons in either that photoconductor binder material or thepigment materials depending on which the radiation falls. Since most ofthe carriers are created at or near the surface of the photoinsulatingmember charge transport presents no serious problem. Therefore at point(A) light has caused the photogeneration of an electron and a hole inthe photoconductor and at point (B) photogeneration takes place in thepigment. As can be seen from the illustration, in order for the pigmentto have its effect in increasing the sensitivity of theelectrophotographic member it generally has to be present in arelatively large concentration and be at or near the surface of thephotoreceptor. This is to be contrasted with FIG. 4 wherephotogeneration takes place exclusively in the photoconductive layer,the active transport layer being substantially transparent to theincident radiation, and the photoconductor material is well protected bysaid active layer there being no requirement that the photoconductor beat or near the surface of the photoreceptor member. Furthermore, it canbe seen in FIG. 5 that, in order for the pigment to function in themember, a significant amount must be kept on or at the surface where itis prone to inevitable abrasion and exposure to the atmosphere.

FIG. 6 offers by further contrast an illustration of a photoreceptor ofthe prior art in which pigment 12 is dispersed in an inert resinmaterial 13 in two different concentrations, A and B. Because there isno photogeneration in the resin binder it is generally necessary thatthe photoconductive pigment or dye be in sufficient concentration orgeometric proximity to support charge injection throughout the bindersystem. Hence, as can be seen, in section (A) where there is a largeconcentration of pigment impinging light 14 creates a photogeneratedhole and electron pair which is then transported through the pigments tothe positively charged surface while in section (B), where theconcentration of the pigment is insufficient to effectparticle-to-particcle proximity impinging light creates the electron andhole pair which remains trapped because of failure of the binder systemto transport the photogenerated charges either to other pigmentparticles or the charged surface. Again this figure is to be contrastedwith FIG. 4 where particle-to-particle proximity or contact of thephotoconductor is unnecessary in the active matrix structure. Inaddition, because particle-to-particle contact is necessary in the inertbinder structure of FIG. 6 resolution problems occur because thegeometry of the particle may not correspond to the direction of theimpinging light thereby resulting in irregular dissipation of thecharge.

When the two layered configuration of photoconductor and activetransport material has sufficient strength to form a self supportingmember (termed pellicle), it is possible to eliminate the physical baseor support member and substitute therefore any of the variousarrangement well known in the art, in place of the ground planepreviously supplied by the base layer. A ground plane, in effect,provides a source of image charges of both polarities. The deposition ofthe insulating two layered structure of the present invention ofsensitizing charges of the desired polarity cause those charges in theground plane of opposite polarity to migrate to the interface at thephotoconductive insulating layer. Without this capacity of theinsulating member by itself would be such that it could not acceptenough charge to sensitize the layer to a xerographically usefulpotential. It is the electrostatic field between the deposited chargeson one side of the xerographic two layered member and the inducedcharges (from the ground plane) on the other side that stresses thexerographic member so that when an electron is excited (in thephotoconductive layer) to the conduction band by a photon therebycreating a hole-electron pair, the charges migrate under the influenceof this field thereby creating the latent electrostatic image. Thereforeit is obvious that if the physical ground plane is omitted a substitutetherefore may be provided by depositing on opposite sides of the twolayered xerographic insulating pellicle, simultaneously, electrostaticcharges of opposite polarity. Thus if positive electrostatic charges areplaced on one side of the pellicle as by corona charging as described inUS. Pat. No. 2,777,957 to L. E. Walkup, the simultaneous deposition ofnegative charges on the other side of the pellicle also by coronacharging will created an induced, that is, a virtual, ground planewithin the body of the pellicle just as if the charges of oppositepolarity has been supplied to the interface by being induced from anactual ground plane. Such an artificial ground plane permits theacceptance of a useable sensitizing charge and at the same time permitsmigration of charges under the applied field when exposed to activatingradiation. As used hereinafter in the specification and claims the termconductive base includes both a physical base and an artificial one asdescribed herein.

The physical shape of the xerographic active transport plate may be inthe form whatsoever as desired by the formulator such as a flat,spherical, cylindrical plate, etc. The plate may be flexible or rigid asdesired.

DESCRIPTION OF PREFERRED EMBODIMENT For purposes of affording thoseskilled in the art a better understanding of the invention, thefollowing illustrative examples are given:

EXAMPLE I A photosensitive layered structure plate similar to that shownin FIG. 1 is prepared by the following technique: An aluminum substrateis dip coated with a three percent duPont Zytel nylon-denatured alcoholsolution to form a 0.2 micron thick blocking layer. The coated substrateis then dried for approximately 30 minutes. Thereafter a one micronlayer of amorphous selenium is vacuum evaporated onto the blocking layerby conventional vacuum techniques such as those disclosed by Bixby inUS. Pat. Nos. 2,753,278 and 2,970,906. The selenium coated substrate isthen cooled to 0C and a l0micron layer of 2,4,7-trinitro-9- fluorenone(TNF) is vacuum evaporated onto. the amorphous selenium layer.

The TNF overcoated plate is then placed in a Xerox Model D Machine wherea copy of an original is made by positive corona charging the plate to avalue of 800 volts and exposing the original to radiation in thewavelength region of 4200 to 6500 Angstrom Units whereby an image isformed on the plate. The image is then developed and transferred topaper whereby a reproduction of the original is accomplished. The copyis of excellent quality being comparable to copies made on aconventional amorphous selenium electrophotographic plate. In additionthe active transport photoreceptor can be recycled for multiple copiesand its organic surface is easily cleaned.

EXAMPLE n A TNF active transport electrophotographic plate is preparedin a manner similar to that outline in Example I except that a 2 micronlayer of the [3 form of metal- 13 free phthalocyanine, an organicphotoinjecting pigment, is applied on top of the blocking layer to forma 0.5 micron photoconductive layer by dip coating the substrate blockinglayer in a solution of the phthalocyanine pigment, dioxane, anddichloromethane and alrowing the coating to dry for several hours.-After drying, a 20 micron layer of dinitroacridine is then vacuumevaporated in the same manner as Example I to form the active transportoverlayer.

The resulting electrophotographic plate is then placed in a Model DXerographic Copy Machine where copy is made in the same manner asExample I by positive corona charging to 800 volts and exposing in awavelength region of 4200 to 6500 Angstrom Units. The resulting recycledcopies have as good a quality of reproduction as that prepared inExample I.

EXAMPLE III the grams of Lexan, a polycarbonate resin, were stirred intoa solvent blend of 40 grams dioxane and 40 grams dichloromethane. Tothis solution ten grams of 2,4,7-trinitro-9-fluorenone (TNF) is added.Stirring is continued until solution is complete.

A layered structure is prepared in the same manner as Example I by dipcoating a blocking layer-substrate arrangement in a copperphthalocyanine-solvent composition whereby a 3 micron phthalocyaninelayer is formed. The layered phthalocyanine plate is then dip coated inthe Lexan-TNF solution to form a 10 micron layer of the resin-TNFcomposition. The resulting layered structure is dried for a period of 24hours.

The resin-TNF layered structure is then placed in a Xerox Model DMachine where a copy is made in the same manner as the plate of ExampleI. The quality of reproduction is equivalent to those in Examples I andII which indicates that charge carriers are transported across theresin-TNF layer. Therefore the electron fi'ansport characteristics arenot hindered by placing sufficient quantities of TNF, or any otherelectron transport material, in an electronically inert binder.

EXAMPLE IV A photosensitive layered plate substantially similar to thatshown in FIG. 1 is prepared by the following technique: A glasssubstrate having a conductive layer of tin oxide was vacuum coated witha one micron layer of amorphous selenium by conventional vacuumtechniques such as those disclosed by Bixby in U.S. Pat. Nos. 2,753,278and 2,970,906. A stock solution of 3.32 g. of duPont 49,000 polyesteradhesive and 11.25g. of 2,4,7-trinitro-9-fluorenone was prepared bydissolving these quantities of materials in 58g. of tetrahydrofuran. Theselenium coated substrate was overcoated with the described stocksolution to form a 23 micron thick charge transport layer having apercentage of 2,4,7- trinitro-9-fluorenone in the film after drying of75 percent by weight.

To evaluate the sensitivity of the above described plate as compared tocertain structures of the prior art in a xerographic operative mode, aplate was prepared generally as described in U.S. Pat. No. 3,598,582 butspecifically as follows: A glass plate was vacuum coated with aluminumto allow transmission of 9 percent of the incident light, followed bythe sprinkling of a photoconductive pigment over the surface of theplate. The photoconductive pigment employed was 2,6-bis(pN,Ndimethylaminobenzylideneamino )-benzo 1,2- d:5,4-d) bisthiazole andwas chosen as the pigment 14 based on test data shown in U.S. Pat. No.3,489,558 and U.S. Pat. No. 3,501 ,298, these patents being incorporatedby reference in U.S. Pat. No. 3,598,582. This data indicated that thedescribed pigment would most likely be the most sensitive pigment for axerographic operative mode.

After the pigment was applied, it was rubbed unidirectionally until 'adistinct polarization of transmitted light was observed and atransmission density was obtained of about 0.2 to 0.6 as specified inExample I, of U.S. Pat. No. 3,598,582. Following this the plate wasovercoated with a layer of poly-N-vinyl carbazole to a thickness of 14microns and dried.

The plate produced as generally described in U.S. Pat. No. 3,598,582 wasthen charged negatively while the plate produced pursuant to the instantinvention was charged positively. This was necessary for comparativepurposes since the poly-N-vinyl carbazole specitied in U.S. Pat. No.3,598,582 transports holes as opposed to electrons. Following charging,exposure to a substantially unpolarized tungsten light source wascarried out while the surface potential was continuously monitored andthe exposure times for 50 percent discharge of the surface voltage forboth plates were measured. These times are expressed below in Table I ast /2 (50 percent), this being a recognized measure of the xerographicsensitivity of the noted plates.

It may be seen from the listed data and in view of the much fasterdischarge of the plate of the instant invention that it is much moresensitive than the plate of the prior art for xerography. Comparing thelowest field applied to the prior art plate, the plate of the instantinvention is about 16 times faster, while at the highest field applied,it is about 6 times faster.

The present invention has been described with reference to certainspecific embodiments which have been presented in illustration of theinvention. It is to be understood however that numerous variations ofthe invention may be made and that it is intended to encompass suchvariation within the scope and spirit of I the invention as described bythe following claims.

15 from the group consisting of phthalic anhydride, tetrachlorophthalicanhydride, benzil, mellitic anhydride, s-tricyanobenzene, picrylchloride, 2,4- dinitrochlorobenzene, 2,4-dinitrobromobenzene,4-nitrobiphenyl, 4,4-dinitrobiphenyl, 2,4,6-trinitroanisole,trichlorotrinitrobenzene, trinitro-o-toluene 4,6-dichloro- 1,3-dinitrobenzene, 4,6- dibromol ,3-dinitrbe nzene, p-dinitrobenzene,chloranil, bromanil, 2,4,7-trinitro-9-fluorenone,2,4,5,7-tetranitrofluorenone, trinitroanthracene, dinitroacridine,tetracyanopyrene, and dinitroanthraquinone; the photoconductive layerhaving a thickness between 0.02 and 25 microns and dispersed in from 0to 95 volume percent binder but comprising at least 25 volume percentwhen an electronically inert binder is employed, the photoconductorbinder thickness ranging from 0.05 to 20 microns when a binder isemployed; the transport material, if dispersed in an electronicallyinert binder, is present in a volume ratio of at least 25 percent activetransport material to electronically inert binder;

uniformly positive charging said plate, and c. exposing said plate to asource of radiation in the wavelength region of from about 4200 to 6500Angstroms to which the active layer is substantially transparent andnon-absorbing whereby injection and transport of photogeneratedelectrons from said photoconductive layer occurs through said activetransport layer to form a latent electrostatic image on the surface ofsaid plate.

2. The method of claim 1 wherein the photoconductive layer comprises amaterial selected from the group consisting of vitreous selenium,amorphous selenium, selenium alloys, trigonal selenium, cadmiumsulfoselenide, cadmium sulfide, cadmium selenide, zinc oxide, andmixtures thereof.

3. The method of'claim l which further includes developing said latentimage to make it visible.

4. The method of claim 1 in which the substrate is substantiallytransparent and exposure is carried out through said substrate.

5. An electrophotographic plate comprising in successive layers:

a. a conductive substrate,

b. a blocking layer,

c. an inorganic photoconductive layer, and

d. an organic charge transport layer consisting essentially of2,4,7-trinitor-9-fluorenone; the photoconductive layer having athickness between 0.02 and 25 microns and dispersed in from 0 to volumepercent binder but comprising at least 25 volume percent when anelectronically inert binder is employed, the photoconductor binderthickness ranging from 0.05 to 20 microns when a binder is employed; thetransport material, if dispersed in an electronically inert binder, ispresent in a volume ratio of at least 25 percent active transportmaterial to electronically inert binder.

6. An electrophotographic plate as claimed in claim 5 wherein the chargetransport layer consists essentially of about 50% by weight of2,4,7-trinitro-9-fluorenone in a resin binder.

7. An electrophotographic plate as claimed in claim 5 wherein thebarrier layer is aluminum oxide.

8. An electrophotographic plate as claimed in claim 5 wherein theinorganic photoconductive layer comprises a material selected from thegroup consisting of cadmium sulfide, cadmium selenide or zinc sulfide.

9. The method of imaging of claim 1 wherein the transport material isdispersed in an electronicallyv inert binder in a volume ratio of atleast 25 percent active transport material to electronically inertbinder.

10. An electrophotographic plate as claimed in claim 5 wherein thetransport material is dispersed in an electronically inert binder in avolume ratio of at least 25 percent active transport material toelectronically inert binder.

UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTIONPATENT NO. 1 3 92 034 DATED i December 23, 1975 INVENTOR(S) Paul J.Regensburger 9 it is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 2, line 22, cancel "reflux" and substitute therefore reflex QColumn 4, line 59, cancel "filed" and substitute therefore field Column7, line 29, cancel "becuase" and substitute therefore because Q Column9, line 36, cancel" the" and substitute therefore that Column ll, line36, cancel "particcle" and substitute therefore particle Q Column 12,line 15, cancel created" and substitute therefore create Column 12, line27, cancel "the and substitute therefore any g Column 12, line 67,cancel "outline" and substitute therefore outlined Engned and Sealedthis thirtieth D f March 1976 O [SEAL] Arrest:

RUTH C. MASON C. MARSHALL DANN Anestmg 1 ('mnmissivm'r ofParenIs andTrademarks

1. A method of imaging which comprises: a. providing a xerographic platehaving a supporting substrate, an unoriented inorganic photoconductivelayer overlaying said substrate, and an electronically active transportlayer overlaying said photoconductive layer, said photoconductive layercomprising a photoconductor having the capability of photogeneratingelectrons and injecting them into an adjacent active transport layer,said active layer having the capability of supporting the injection ofelectrons and transporting said electrons through said active materialwherein said active material comprises at least one material selectedfrom the group consisting of phthalic anhydride, tetrachlorophthalicanhydride, benzil, mellitic anhydride, s-tricyanobenzene, picrylchloride, 2,4-dinitrochlorobenzene, 2,4-dinitrobromobenzene,4-nitrobiphenyl, 4,4-dinitrobiphenyl, 2,4, 6-trinitroanisole,trichlorotrinitrobenzene, trinitro-o-toluene4,6-dichloro-1,3-dinitrobenzene, 4,6-dibromo-1,3-dinitrobenzene,p-dinitrobenzene, chloranil, bromanil, 2,4,7-trinitro-9-fluorenone,2,4,5,7-tetranitrofluorenone, trinitroanthracene, dinitroacridine,tetracyanopyrene, and dinitroanthraquinone; the photoconductive layerhaving a thickness between 0.02 and 25 microns and dispersed in from 0to 95 volume percent binder but comprising at least 25 volume percentwhen an electronically inert binder is employed, the photoconductorbinder thickness ranging from 0.05 to 20 microns when a binder isemployed; the transport material, if dispersed in an electronicallyinert binder, is present in a volume ratio of at least 25 percent activetransport material to electronically inert biNder; b. uniformly positivecharging said plate, and c. exposing said plate to a source of radiationin the wavelength region of from about 4200 to 6500 Angstroms to whichthe active layer is substantially transparent and non-absorbing wherebyinjection and transport of photogenerated electrons from saidphotoconductive layer occurs through said active transport layer to forma latent electrostatic image on the surface of said plate.
 2. The methodof claim 1 wherein the photoconductive layer comprises a materialselected from the group consisting of vitreous selenium, amorphousselenium, selenium alloys, trigonal selenium, cadmium sulfoselenide,cadmium sulfide, cadmium selenide, zinc oxide, and mixtures thereof. 3.The method of claim 1 which further includes developing said latentimage to make it visible.
 4. The method of claim 1 in which thesubstrate is substantially transparent and exposure is carried outthrough said substrate.
 5. AN ELECTROPHOTOGRAPHIC PLATE COMPRISING INSUCCESSIVE LAYERS: A. A CONDUCTIVE SUBSTRATE, B. A BLOCKING LAYER, C. ANINORGANIC PHOTOCONDUCTIVE LAYER, AND D. AN ORGANIC CHARGE TRANSPORTLAYER CONSISTING ESSENTIALLY OF 2,4,7-TRINITOR-9-FLUORENONE; THEPHOTOCONDUCTIVE LAYER HAVING A THICKNESS BETWEEN 0.02 AND 25 MICRONS ANDDISPERSED IN FROM 0 TO 95 VOLUME PERCENT BINDER BUT COMPRISING AT LEAST25 VOLUME PERCENT WHEN AN ELECTRONICALLY INERT BINDER IS EMPLOYED, THEPHOTOCONDUCTOR BINDER THICKNESS RANGING FROM 0.05 TO 20 MICRONS WHEN ABINDER IS EMPLOYED; THE TRANSPORT MATERIAL, IF DISPERSED IN ANELECTRONICALLY INERT BINDER, IS PRESENT IN A VOLUME RATIO OF AT LEAST 25PERCENT ACTIVE TRANSPORT MATERIAL TO ELECTRONICALLY INERT BINDER.
 6. Anelectrophotographic plate as claimed in claim 5 wherein the chargetransport layer consists essentially of about 50% by weight of2,4,7-trinitro-9-fluorenone in a resin binder.
 7. An electrophotographicplate as claimed in claim 5 wherein the barrier layer is aluminum oxide.8. An electrophotographic plate as claimed in claim 5 wherein theinorganic photoconductive layer comprises a material selected from thegroup consisting of cadmium sulfide, cadmium selenide or zinc sulfide.9. The method of imaging of claim 1 wherein the transport material isdispersed in an electronically inert binder in a volume ratio of atleast 25 percent active transport material to electronically inertbinder.
 10. An electrophotographic plate as claimed in claim 5 whereinthe transport material is dispersed in an electronically inert binder ina volume ratio of at least 25 percent active transport material toelectronically inert binder.